WASHINGTON — An asteroid half the size of a football field passed within an astronomical hair’s breadth of Earth Friday in the closest such encounter in a century.

The asteroid, called 2012 DA14, came within 17,200 miles of Earth at 2:25 p.m. Washington time, over Indonesia. Telescope images from western Australia, broadcast on NASA Television, showed the asteroid as a white speck against the blackness of space.

While DA14 didn’t hit the planet, astronauts and interplanetary evangelists say its fly-by — and a meteor strike in Russia early Friday — serve as evidence that monitoring for risks from space objects must be increased.

“If this asteroid were to hit London, the entire metropolitan area would be gone,” Sergio Camacho, head of the effort at the Vienna-based U.N. Office for Outer Space Affairs, said of DA14 in an interview.

Camacho’s “Action Team on Near-Earth Objects” is set to propose a global asteroid warning network, which would discover, monitor and characterize the risks of those orbiting objects. The group also suggests a team to oversee a space mission to deflect or blow up an asteroid headed for Earth.

An asteroid of a similar size slammed into rural Russia in 1908 and leveled millions of trees over 820 square miles. The asteroid scientists say plowed into Earth about 66 million years ago, wiping out the dinosaurs, was about six miles in diameter.

Other space objects, such as the meteor that slammed into Russia’s Urals region Friday and caused hundreds of injuries, couldn’t be detected with existing telescopes, said Detlef Koschny, a scientist at the European Space Agency who is also part of the U.N. working group.

“The goal is that we can see objects this size about two days before they hit the Earth,” he said Friday in a telephone interview. Information on those smaller objects should also eventually be part of a U.N. global database, he said.

In 1998, the U.S. space agency NASA began working on finding and tracking the largest asteroids, typically more than one kilometer in diameter and capable of destroying much of humanity. That’s left a big gap in finding smaller objects that would demolish a city while sparing the rest of civilization.

NASA says it has found and mapped 1,310 of the largest, most dangerous “near-Earth objects.” The total may account for less than 10 percent of all space threats, it says.

“The whole field needs more resources and funding,” Bruce Betts, director of projects at the Pasadena, California-based Planetary Society, founded in 1980 by Carl Sagan to promote space exploration, said in an interview.

In 2008, a group of former astronauts submitted a report to the U.N. working group, urging a global response to asteroid threats. “Questions arise regarding the authorization and responsibility to act, liability, and financial implications,” the Association of Space Explorers wrote in the paper.

In its final report, the U.N. group proposes an international asteroid warning network to be a clearing house for all observations of near-Earth objects, the technical term for asteroids in close vicinity to the planet.

DA14 was discovered in February 2012 by a group of amateur astronomers at La Sagra Observatory in southern Spain. Jaime Nomen, a dental surgeon who dabbled in astronomy, said his group bought a high-powered telescopic camera and software with the help of a $7,695 grant in 2010 from the Planetary Society.

The probability of an asteroid hitting Earth is fairly low.

“If we’re lucky, none of us will see an asteroid coming toward the Earth in our lifetime,” Camacho said. “But if we’re not lucky and we didn’t do anything, the only thing we might be able to do is evacuate.”

“This alone could be a disastrous event.”

An asteroid on course to hit Earth can be deflected with a spacecraft, redirected with a “gravity tractor” hovering nearby or, as a last resort, targeted with a nuclear explosion.

In the meantime, bureaucracy needs to work at its pace. The U.N. group’s recommendations must be endorsed by the General Assembly, which Camacho expects will consider it when it meets in New York in October.

For some asteroids, there could be 20 to 30 years until the threat becomes imminent, Camacho said. “But we could find one that would give us three months.”

Koschny said the asteroid and meteor encounters should help the U.N. group’s cause: “This shows that we are doing something that should be taken seriously. It’s not only something a crazy scientist came up with.”

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Weber reported from Vienna.

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